This invention relates generally to an electrotherapy method and apparatus for delivering an electrical pulse to a patient""s heart. In particular, this invention relates to a method and apparatus for tailoring a second phase of biphasic waveform delivered by an external defibrillator, to random patients, by performing intelligent calculations and analysis to the results of a first phase segment of a biphasic defibrillation waveform and other parameters pertaining thereto based on theory and practice as disclosed herein.
Devices for defibrillating a heart have been known for sometime now. Implantable defibrillators are well accepted by the medical community as effective tools to combat fibrillation for an identified segment of the population. A substantial amount of research in fibrillation and the therapy of defibrillation has been done. Much of the most recent research has concentrated on understanding the effects that a defibrillation shock pulse has on fibrillation to terminate such a condition.
A monophasic waveform is defined to be a single phase, capacitive-discharge, time-truncated, waveform with exponential decay. A biphasic waveform is defined to comprise two monophasic waveforms, separated by time and of opposite polarity. The first phase is designated and the second phase is designated xcfx862. The delivery of xcfx861is completed before the delivery of xcfx862 is begun.
After extensive testing, it has been determined that biphasic waveforms are more efficacious than monophasic waveforms. There is a wide debate regarding the reasons for the increased efficacy of biphasic waveforms over that of monophasic waveforms. One hypothesis holds that xcfx861 defibrillates the heart and xcfx862 performs a stabilizing action that keeps the heart from refibrillating.
Biphasic defibrillation waveforms are now the standard of care in clinical use for defibrillation with implantable cardioverter-defibrillators (ICDs), due to the superior performance demonstrated over that of comparable monophasic waveforms. To better understand these significantly different outcomes, ICD research has developed cardiac cell response models to defibrillation. Waveformn design criteria have been derived from these first principles and have been applied to monophasic and biphasic waveforms to optimize their parameters. These principles-based design criteria have produced significant improvements over the current art of waveforms.
In a two-paper set, Blair developed a model for the optimal design of a monophasic waveform when used for electrical stimulation. (1) Blair, H. A., xe2x80x9cOn the Intensity-time relations for stimulation by electric currents.xe2x80x9d I. J. Gen. Physiol. 1932; 15: 709-729. (2) Blair, H. A., xe2x80x9cOn the Intensity-time Relations for stimulation by electric currents.xe2x80x9d II. J. Gen. Physiol. 1932; 15: 731-755. Blair proposed and demonstrated that the optimal duration of a monophasic waveform is equal to the point in time at which the cell response to the stimulus is maximal. Duplicating Blair""s model, Walcott extended Blair""s analysis to defibrillation, where they obtained supporting experimental results. Walcott, et al., xe2x80x9cChoosing the optimal monophasic and biphasic waveforms for ventricular defibrillation.xe2x80x9d J. Cardiovasc Electrophysiol. 1995; 6: 737-750.
Independently, Kroll developed a biphasic model for the optimal design of xcfx862 for a biphasic defibrillation waveform. Kroll, M. W., xe2x80x9cA minimal model of the single capacitor biphasic defibrillation waveform.xe2x80x9d PACE 1994; 17:1782-1792. Kroll proposed that the xcfx862 stabilizing action removed the charge deposited by xcfx861 from those cells not stimulated by xcfx861. This has come to be known as xe2x80x9ccharge burpingxe2x80x9d. Kroll supported his hypothesis with retrospective analysis of studies by Dixon, et al., Tang, et al., and Freese, et al. regarding single capacitor, biphasic waveform studies. Dixon, et al., xe2x80x9cImproved defibrillation thresholds with large contoured epicardial electrodes and biphasic waveforms.xe2x80x9d Circulation 1987; 76:1176-1184; Tang, et al. xe2x80x9cVentricular defibrillation using biphasic waveforms: The Importance of Phasic duration.xe2x80x9d J. Am. Coll. Cardiol. 1989; 13:207-214; and Feeser, S. A., et al. xe2x80x9cStrength-duration and probability of success curves for defibrillation with biphasic waveforms.xe2x80x9d Circulation 1990; 82: 2128-2141. Again, the Walcott group retrospectively evaluated their extension of Blair""s model to xcfx862 using the Tang and Feeser data sets. Their finding further supported Kroll""s hypothesis regarding biphasic defibrillation waveforms. For further discussions on the development of these models, reference may be made to PCT publications WO 95/32020 and WO 95/09673 and to U.S. Pat. No. 5,431,686.
The charge burping hypothesis can be used to develop equations that describe the time course of a cell""s membrane potential during a biphasic shock pulse. At the end of xcfx861, those cells that were not stimulated by xcfx861 have a residual charge due to the action of xcfx861 on the cell. The charge burping model hypothesizes that an optimal pulse duration for xcfx862 is that duration that removes as much of the xcfx861 residual charge from the cell as possible. Ideally, these unstimulated cells are set back to xe2x80x9crelative ground.xe2x80x9d The charge burping model proposed by Kroll is based on the circuit model shown in FIG. 2b which is adapted from the general model of a defibrillator illustrated in FIG. 2a. 
The charge burping model also accounts for removing the residual cell membrane potential at the end of a xcfx861 pulse that is independent of a xcfx862. That is, xcfx862 is delivered by a set of capacitors separate from the set of capacitors used to deliver xcfx861. This charge burping model is constructed by adding a second set of capacitors, as illustrated in FIG. 3. In this figure, Cl represents the xcfx861 capacitor set, C2 represents the xcfx862 capacitor set RH represents the resistance of the heart, and the pair CM and RM represent membrane series capacitance and resistance of a single cell. The node VS represents the voltage between the electrodes, while VM denotes the voltage across the cell membrane.
External defibrillators send electrical pulses to the patient""s heart through electrodes applied to the patient""s torso. External defibrillators are useful in any situation where there may be an unanticipated need to provide electrotherapy to a patient on short notice. The advantage of external defibrillators is that they may be used on a patient as needed, then subsequently moved to be used with another patient.
However, this important advantage has two fundamental limitations. First, external defibrillators do not have direct contact with the patient""s heart. External defibrillators have traditionally delivered their electrotherapeutic pulses to the patient""s heart from the surface of the patient""s chest. This is known as the transthoracic defibrillation problem. Second, external defibrillators must be able to be used on patients having a variety of physiological differences. External defibrillators have traditionally operated according to pulse amplitude and duration parameters that can be effective in all patients. This is known as the patient variability problem.
The prior art described above effectively models implantable defibrillators, however it does not fully address the transthoracic defibrillation problem nor the patient variability problem. In fact, these two limitations to external defibrillators are not fully appreciated by those in the art. For example, prior art disclosures of the use of truncated exponential monophasic or biphasic shock pulses in implantable or external defibrillators have provided little guidance for the design of an external defibrillator that will successfully defibrillate across a large, heterogeneous population of patients. In particular, an implantable defibrillator and an external defibrillator can deliver a shock pulse of similar form, and yet the actual implementation of the waveform delivery system is radically different.
In the past five years, new research in ICD therapy has developed and demonstrated defibrillation models that provide waveform design rules from first principles. These defibrillation models and their associated design rules for the development of defibrillation waveforms and their characteristics were first developed by Kroll and Irnich for monophasic waveforms using effective and rheobase current concepts. (1) Kroll, M. W., xe2x80x9cA minimal model of the monophasic defibrillation pulse.xe2x80x9d PACE 1993; 15: 769. (2) Irnich, W., xe2x80x9cOptimal truncation of defibrillation pulses.xe2x80x9d PACE 1995; 18: 673. Subsequently, Kroll, Walcott, Cleland and others developed the passive cardiac cell membrane response model for monophasic and biphasic waveforms, herein called the cell response model. (1) Kroll, M. W., xe2x80x9cA minimal model of the single capacitor biphasic defibrillation waveform.xe2x80x9d PACE 1994; 17: 1782. (2) Walcott, G. P., Walker, R. G., Cates. A. W., Krassowska, W., Smith, W. M, Ideker RE. xe2x80x9cChoosing the optimal monophasic and biphasic waveforms for ventricular defibrillation.xe2x80x9d J Cardiovasc Electrophysiol 1995; 6:737; and Cleland BG. xe2x80x9cA conceptual basis for defibrillation waveforms.xe2x80x9d PACE 1996; 19:1186).
A significant increase in the understanding of waveform design has occurred and substantial improvements have been made by using these newly developed design principles. Block et al. has recently written a comprehensive survey of the new principles-based theories and their impact on optimizing internal defibrillation through improved waveforms. Block M, Breithardt G., xe2x80x9cOptimizing defibrillation through improved waveforms.xe2x80x9d PACE 1995; 18:526.
There have not been significant developments in external defibrillation waveforms beyond the two basic monophasic waveforms: the damped sine or the truncated exponential. To date, their design for transthoracic defibrillation has been based almost entirely on empirically derived data. It seems that the design of monophasic and biphasic waveforms for external defibrillation has not yet been generally influenced by the important developments in ICD research.
Recently there has been reported research on the development and validation of a biphasic truncated exponential waveform in which it was compared clinically to a damped sine waveform. For additional background, reference may be made to U.S. Pat. Nos. 5,593,427, 5,601,612 and 5,607,454. See also: Gliner B. E., Lyster T. E., Dillon S. M., Bardy G. H., xe2x80x9cTransthoracic defibrillation of swine with monophasic and biphasic waveforms.xe2x80x9d Circulation 1995; 92:1634-1643; Bardy G. H., Gliner B. E., Kudenchuk P. J., Poole J. E., Dolack G. L., Jones G. K., Anderson J., Troutman C., Johnson G.; xe2x80x9cTruncated biphasic pulses for transthoracic defibrillation.xe2x80x9d Circulation 1995; 91:1768-1774; and Bardy G. H. et al, xe2x80x9cFor the Transthoracic Investigators. Multicenter comparison of truncated biphasic shocks and standard damped sine wave monophasic shocks for transthoracic ventricular defibrillation.xe2x80x9d Circulation 1996; 94:2507-2514. Although the research determined a usable biphasic waveform, there was no new theoretical understanding determined for external waveform design. It appears that external waveform research may develop a xe2x80x9crules-of-thumb by trial and errorxe2x80x9d design approach much like that established in the early stages of theoretical ICD research. The noted limitations of the transthoracic biphasic waveform may be due in part to a lack of principles-based design rules to determine its waveform characteristics.
There is a continued need for a device designed to perform a quick and automatic adjustment of phase 2 relative to phase 1 if AEDs are to be advantageously applied to random patients according to a cell response model. Further, the model and the device must be adaptable to patient variance and be able to provide automatic adjustment in a dynamic environment.
The present invention relates to an external defibrillation method and apparatus that addresses the limitations in the prior art. The present invention incorporates three singular practices that distinguish the practice of designing external defibrillators from the practice of designing implantable defibrillators. These practices are 1) designing multiphasic transthoracic shock pulse waveforms from principles based on cardiac electrophysiology, 2) designing multiphasic transthoracic shock pulse waveforms in which each phase of the waveform can be designed without implementation limitations placed on its charging and delivery means by such means for prior waveform phases, and 3) designing multiphasic transthoracic shock pulse waveforms to operate across a wide range of parameters determined by a large, heterogeneous population of patients.
In particular, the present invention provides for a method and apparatus for tailoring and reforming a second phase (xcfx862) of a biphasic defibrillation waveform relative to a first phase (xcfx861) of the waveform based on intelligent calculations. The method includes the steps of determining and providing a quantitative description of the desired cardiac membrane response function. A quantitative model of a defibrillator circuit for producing external defibrillation waveforms is then provided. Also provided is a quantitative model of a patient which includes a chest component, a heart component and a cell membrane component. A quantitative description of a transchest external defibrillation waveform that will produce the desired cardiac membrane response function is then computed. Intelligent calculations based on the phase 1 cell response is then computed to determine the desired phase 2 waveform. The computation is made as a function of the desired cardiac membrane response function, the patient model and the defibrillator circuit model.